Improve IPv6 reassembly performance by hashing fragments into buckets.

Currently, all IPv6 fragment reassembly queues are kept in a flat
linked list. This has a number of implications. Two significant
implications are: all reassembly operations share a common lock,
and it is possible for the linked list to grow quite large.

Improve IPv6 reassembly performance by hashing fragments into buckets,
each of which has its own lock. Calculate the hash key using a Jenkins
hash with a random seed.

Reviewed by: jhb
Security: FreeBSD-SA-18:10.ip
Security: CVE-2018-6923